PFEL Coastal Upwelling Indices

How PFEL Determines the Upwelling Indices

PFEL coastal upwelling indices are calculated based upon Ekman's theory of mass transport due to wind stress. Assuming homogeneity, uniform wind and steady state conditions, the mass transport of the surface water due to wind stress is 90° to the right of the wind direction in the Northern Hemisphere. Ekman mass transport is defined as the wind stress divided by the Coriolis parameter (a function of the earth's rotation and latitude). The depth to which an appreciable amount of this offshore transport occurs is termed the surface Ekman layer, and is generally 50 to 100 meters deep.

Ekman transports are resolved into components parallel and normal to the local coastline orientation. The magnitude of the offshore component is considered to be an index of the amount of water upwelled from the base of the Ekman layer. Positive values are, in general, the result of equatorward wind stress. Negative values imply downwelling, the onshore advection of surface waters accompanied by a downward displacement of water.

Historically, the indices were computed from monthly mean pressure fields prepared by FNMOC on a 3 mesh grid. After providing PFEL with several alternate pressure field grids over time, FNMOC currently produces six-hourly fields of surface pressure on a global spherical 1 mesh (a 180 x 360 grid). The standard west coast six-hourly upwelling indices are a product of the 3 pressure field interpolated from the 1 grid. Monthly indices are derived from a 3 mesh that is interpolated from the monthly-averages of the six-hourly 1 pressures.

Monthly Processing

Changes to Processing for Upwelling Indices and Related Data Products (December 1996)

On a monthly basis, the Pacific Fisheries Environmental Laboratory generates indices of the intensity of large-scale, wind-induced coastal upwelling at 15 standard locations along the west coast of North America. The indices are based on estimates of offshore Ekman transport driven by geostrophic wind stress. Geostrophic winds are derived from six-hourly synoptic and monthly mean surface atmospheric pressure fields. The pressure fields are provided by the U.S. Navy Fleet Numerical Meteorological and Oceanographic Center (FNMOC), Monterey, CA. The idea behind the upwelling indices was to develop simple time series that represent variations in coastal upwelling. Daily and monthly index time series are provided regularly to scientists and managers concerned with marine ecosystems and their biota, and have been used in scores of studies and scientific publications.